Digital networks are used to transfer computer data and other signals between computers and other end apparatus. In connecting home users to digital networks, analog lines are commonly used. In transmitting data over analog lines, modems on opposite sides of the analog lines modulate the transmitted signals on one side and demodulate the transmitted signals on the other side.
In order to allow the end modems to communicate, protocols define the precise signals to be transmitted by the modems. These protocols include, among others, voice band modem (VBM) protocols which use the frequency band allocated to voice signals and DSL protocols (e.g., ADSL) which use-much larger portions of the frequency band of the analog line.
Some of the defined protocols are very complex, so as to achieve maximal utilization of the available bandwidth. Due to the complexity of the modem protocols, there are cases in which different vendors implement the protocols differently, leading in some cases to problems in communications.
Modem emulators are sometimes used to test the operation of modems. A modem emulator is connected on an opposite side of a communication line to which the tested modem is connected, transmits testing signal to the tested modem and provides a report on the operation of the tested modem. Another existing tool for testing modems is a pair of scope modems which are connected in the middle of a tested analog line connecting opposite end modems. Each modem in the pair of scope modems connects to a different one of the opposite end modems, forming a double connection. The pair of scope modems mirror data between the connections and provide the transferred data to an upper layer for analysis.
For security reasons it may be desired to listen to communication lines of suspected individuals. When the communication lines carry modem signals, the signals are generally demodulated in order to determine their content. The demodulation is a relatively hard task, especially if the demodulation is performed by a passive unit, which cannot request retransmissions and cannot adjust the transmission levels according to the noise level in the vicinity of the eavesdropping apparatus. Therefore, when possible, eavesdropping is performed on signals which are not modulated (e.g., already demodulated). Most VBM connections are established with an Internet Service provider (ISP) and eavesdropping can be performed by the ISP. Modem connections, however, can be established between two private modems. In such connections, it is not always known, for a monitored modem, where the other end modem is and it is not always possible to access that modem.
The signals of ADSL connections, on the other hand, are always demodulated by a modem at a local line-card of a telephone service company, and when necessary can be monitored at the line-card.